Crystalline aluminosilicates are generally known as zeolites, and both natural and synthetic products are hydrated aluminosilicates having a crystal structure composed fundamentally of three-dimensional frameworks; one of which is consisting of SiO.sub.4 tetrahedra formed by coordinating four oxygen atoms at the apexes of a tetrahedron around the central silicon (Si) atom and the other three-dimensional framework is AlO.sub.4 tetrahedra formed by replacing the silicon atom in above SiO.sub.4 tetrahedra with an aluminum (Al) atom.
It is known that the SiO.sub.4 tetrahedra and the AlO.sub.4 tetrahedra constitute basic units consisting of 4-, 5-, 6-, 8-, or 12-membered units formed from 4, 5, 6, 8, or 12 joined tetrahedra or basic units consisting of double rings formed from two such 4-, 5-, 6-, 8-, or 12-membered rings, and that the framework of a crystalline aluminosilicate is determined by interconnection of these basic units.
There are certain cavities in the above framework structure, and their apertures are formed with 6-, 8-, 10- or 12-membered rings. Such cavities have a uniform diameter so that the molecules smaller than a certain size are adsorbed into them while the larger ones can not get into them. Hence, such cristalline aluminosilicates are known as "molecularsieves" according to their functions, and used as adsorbents, catalysts for chemical reactions or catalyst carriers in a wide variety of chemical processes.
In recent years, their applications utilizing a combination of both of the above mentioned functions as a molecular sieve and a catalyst have been energetically studied in a variety of the fields of chemical reactions. These are the so-called molecular shape-selective reaction catalysts and, as shown in the classification made by S. M. Csicsery according to their functions, they are divided into the following three types: (1) catalysts whose active sites can be approached only by special reactions, (2) catalysts in which, among the reactants which have reacted at the active sites, only those having special shapes can leave the reaction sites, and (3) catalysts in which, although individual molecules can freely enter or leave the reaction sites for a bimolecular reaction, they can not react owing to their large transition state ("Zeolite Chemistry and Catalysis" ACS Monograph 171, ACS, Washington D.C., 1976, p. 680).
This classification is made on the basis of only catalytic reactions taking place in the cavities of a crystalline aluminosilicate. Namely, in case of catalytic reactions taking place at active sites on or near the external surface of the crystal, all reactions having low-activation energy can take place--different from the above mentioned catalytic molecular shape selective reactions. Therefore the selectivity of the reactions is lowered.
Accordingly, in order to control the nonselective reactions taking place on or near the external surface of the crystal, there have been proposed a method in which the active sites are buried by coating the crystal surface with a compound and a method in which the solid acidity of the active sites is controlled using some compounds having different solid acidity or alkalinity--such as silicone compounds, phosphorus compounds, magnesium compounds etc.
On the other hand, a method is also known in which the ratio of the number of active sites (the number of active sites in the crystals, having molecular shape selectivity to that of active sites on or near the surface of crystal, having no shape selectivety) is controlled by controlling the crystal size. For example, when the crystal size is increased, the proportion of the active sites in the crystal increases relatively, and the shape selectivity is heightened.
According to this method, however, the access and/or contact of reactants to and/or with active sites are limited relatively which results in lowered overall reaction activity. Conversely, when the crystal size is decreased, the proportion of the active sites on or near the crystal surface increases relatively then the reaction activity is heightened because of a relative increase in the chance for the reactants to approach to or contact with active sites, though the shape selectivity is lowered.
The electrical charge of the aluminum-containing tetrahedron of a crystalline sodium aluminosilicate can be balanced by holding sodium cations within the crystal. It is a well-known theory that these cations can be ion-exchanged by a variety of methods to form a hydrogen-type or a metal-ion exchanged type of crystalline aluminosilicate, which functions as a solid acid catalyst.
In natural crystalline aluminosilicates, the cations are Group I or II metals of the periodic table of the elements, especially sodium, potassium, calcium, magnesium and strontium. Also in synthetic crystalline aluminosilicates, the above metal cations are used, but the use of organic nitrogen cations, for example, quarternary alkylammonium ions such as tetraalkylammonium ions, in addition to these metal cations, has recently been proposed.
For the synthesis of a crystalline aluminosilicate having a high silica/alumina ratio, it has been thought essential to use a nitrogen-containing organic compound as mentioned above as an alkali metal source. The use of the nitrogen-containing organic compounds, however, has disadvantages in that the material cost is high and that the production process is complicated because, in order to employ the obtained synthetic aluminosilicate as a catalyst, it is necessary to remove the nitrogen-containing compounds contained in the product by calcination at high temperatures.
Moreover, in conventional production processes using the above-mentioned tetraalkylammonium compounds or amine compounds such as C.sub.2 to C.sub.10 primary amines, there has been a problem of operation safety because of the latent toxicity of the organic compounds or various dangers accompanying their decomposition or the like encountered in the synthesis, drying and calcination processes.
Furthermore, although it has been proposed to use oxygen containing organic compounds or sulfur containing compounds, these methods can not solve the problems encountered in using the nitrogen-containing organic compounds.
Recently, these problems were partially solved by a invention disclosed in Japan Patent Application No. 143396/1981 (filed Sept. 11, 1981), in which a crystalline aluminosilicate having characteristic crystal structure characterized by an X-ray diffraction pattern had been obtained.
It is a crystalline aluminosilicate having a chemical composition in terms of a molar ratio of oxides of 0.8.about.1.5 M.sub.2 /nO.Al.sub.2 O.sub.3.10.about.100 SiO.sub.2.Z H.sub.2 O, wherein M is a metal cation, n is the valence of the metal, and Z is 0 to 40, and having the powder X-ray diffraction pattern showing at least the interplanar spacings, i.e., d-spacings, shown in Table 1.
TABLE 1 ______________________________________ Interplanar spacings: d (.ANG.) relative intensity (I/Io) ______________________________________ 11.2 .+-. 0.2 S. 10.1 .+-. 0.2 S. 7.5 .+-. 0.15 W. 6.03 .+-. 0.1 M. 3.86 .+-. 0.05 V.S. 3.82 .+-. 0.05 S. 3.76 .+-. 0.05 S. 3.72 .+-. 0.05 S. 3.64 .+-. 0.05 S. ______________________________________
The aluminosilicate having a crystal structure characterized by the above X-ray diffraction pattern was designated as TSZ.
In Table 1, the relative intensities are given in terms of the symbols: V.S.=very strong, S.=strong, M=medium, W.=weak and V.W.=very weak.
From another powder X-ray diffractiometric analysis, it was concluded that TSZ belongs crystallographically to the monoclinic system.
On the other hand, when a zeolite catalyst is applied to an industrial process such as a fluidized bed of gas/oil feedstock or a fluidized operation, for example, in catalytic cracking, the zeolite is supplied in the form of fine particles.
It is desirable to increase the surface area of a catalytically active zeolite as much as possible in view of the fact that only the external surfaces of catalyst particles can be utilized almost exclusively because gas-phase reactions are usually conducted at high space velocities, and the diffusion from the catalyst of surface is limited in liquid phase reactions of heavy oil (U.S. Pat. No. 3,966,644 shows that this diffusion limit is about 1/120 in.). Although it can be improved by reducing the diameters of the catalyst particles, the particles are lowered in strength and collapsed. Therefore, the improvement of catalyst performances by this method is limited, and a zeolite catalyst has been applied to industrial processes, after it was molded into pellets by using a suitable binder. When this method is applied, however, the space velocity of the reactants must be lowered because the utilization rate of a zeolite is lowered, which inevitably lowers the productivity, and in addition, a drawback that the zeolite is poisoned as a result of the movement of the alkali, alkaline earth metal or the like contained in the binder into the zeolite arises. Further, since such pellet-form catalysts are prepared by a process consisting of molding, by compression, a zeolite together with an amorphous binder, the amorphous binder penetrated into the so-called secondary pores present among the zeolite crystals, and therefore neither the quantity nor distribution of secondary pores could be controlled though the physical strength is increased.
Further, the above binder has a limitation in that it must be thermally stable and capable of forming paths for passing gases or liquids as the reactants into zeolite crystals.
A honeycomb-like solid crystal prepared by coating a conventional base with a zeolite crystal was proposed as zeolite catalyst excellent in industrial applications (see, for example, British Patent No. 1,441,443, U.S. Pat. Nos. 3,730,910, 3,468,815, 3,244,643, 3,697,446, etc.), but in none of these, the production is easy and the catalytic activities, the physical strengths, catalyst activity maintenance etc., had room to be improved.
According to U.S. Pat. No. 3,119,660, previously formed metakaolin, either alone or in admixture with zeolite A, is reacted with an alkali solution to form 100%-zeolite A, or a soluble silica source is added to the reaction mixture to form zeolite X or zeolite Y which is used as a constituent of pellets or the like.
Further, a catalyst composition is provided which is prepared by burying a zeolite in a porous matrix and which can provide paths into crystal while minimizing a loss due to abrasion during the operation of the active zeolite crystal (Japan Patent Application-OPI No. 133489/1979).
However, in none of above processes, the production is easy and no consideration is made for the so-called secondary pores present among crystals.
The secondary pores are formed, for example, when crystal powder is molded. Therefore, if these pores can be kept effective without decreasing pellet strength (abbreviated as binderless zeolite), the reactants can easily move from one crystal to another, and in addition, the area of the crystal surface having a catalytic activity is increased practically, with a consequent improvement in the catalytic activity of the pellets.
Accordingly, it is a first object of this invention to provide a catalyst containing TSZ zeolite having effective secondary pores and having a special form excellent in catalytic activity.
It is a second object of this invention to provide a binderless TSZ zeolite excellent as a catalyst used in selectively cracking of n-paraffinic hydrocarbons.
It is a third object of this invention to provide a binderless TSZ zeolite excellent as a catalyst for alkylation reaction of aromatics with alkylating agents such as alcohols or olefins.
It is a fourth object of this invention to provide a process for producing a catalyst containing a TSZ zeolite having effective secondary pores and having a special form excellent in catalytic activity.
It is a fifth object of this invention to provide a process for readily producing a binderless TSZ zeolite of special form.